Nitration of benzene to produce nitrobenzene and dinitrobenzene is commercially important technology. During the production of these compounds, by-product nitrophenols are formed. These nitrophenols are environmentally undesirable being noxious in nature and causing intense coloration of process waste water. In particular, these compounds tie up the oxygen that is necessary to support life in the streams to which the waste discharges. They can cause substantial quantities of suspended matter and excessive coloration at the point of discharge. They can cause an objectionable taste and odor in potable water and fish flesh.
Various biological, physical and chemical methods have beem employed to treat these nitrophenols, which, particularly in the case of the polynitrophenols, are difficult to destroy. The focus of this application is on the chemical methods and particularly on those using hydrogen peroxide.
"Phenolic Problems Solved With Hydrogen Peroxide Oxidation" by Keating et al. at pages 22 to 27 of the December 1978 issue of Industrial Water Engineering teaches the treatment of phenolic wastes, including mononitrophenols and dinitrophenols but not trinitrophenols, using Fenton's Reagent (hydrogen peroxide and ferrous iron). The article concludes that ferric as well as ferrous iron can catalyze oxidation of phenols with hydrogen peroxide, that iron ion levels of from 10-20 .mu.g/gm phenol are recommended when phenolics are no greater than 2000 .mu.g/gm waste water. Higher phenol concentrations require at least 100:1 phenol:iron ratios. Optimum results occur when initial pH of the phenol solution is between 5 and 6. The reaction is carried out at a mole ratio of 3:1 hydrogen peroxide to phenol. At the end of one hour 2,4- and 2,5-dinitrophenols were oxidized by 30% and 73% respectively.
"Hydrogen Peroxide Treats Diverse Wastewaters" by Strunk at pages 32 to 35 of the January/February, 1979, issue of Industrial Wastes teaches that phenol oxidation with hydrogen peroxide in the presence of ferrous salt at a pH of about 4 and a hydrogen peroxide to phenol weight ratio of 1:1 (2.7:1 mole ratio) proceeds rapidly over a 70.degree.-100.degree. F. (21.degree.-38.degree. C.) temperature range. At 120.degree. F. (40.degree. C.), phenol removal efficiency decreases.
United Kingdom Patent Application GB No. 2,056,962A to Pouillot et al. teaches a process for purifying waste water from factories producing dyes or from dye-works. The waste water is stated as containing coloring matter, but treatment of nitrophenols is not taught. The process involves two steps--oxidation with hydrogen peroxide and then coagulation, floculation and decantation. The oxidation step can be catalyzed with a ferrous salt and is preferably carried out at a pH of less than 5.
Japanese Patent Publication No. 77,449/1974 to Okawa et al. teaches a process for the treatment of phenols at a pH of between 2.5 and 3.5 in the presence of ferric chloride followed by treatment with hydrogen peroxide. The pH is then raised to about 10 to precipitate the ferric chloride as ferric hydroxide after which floculents are added and the supernatent liquid is separated from the precipitated matter. Treatment of nitrophenols is not taught.
U.S. Pat. No. 3,637,761 to Kuwata et al. teaches a process for making highly pure trimellitic anhydride from impure trimellitic acid solution containing nitrocompounds, such as picric acid, as impurities. The process first treates the crude trimellitic acid by a conventional reduction method to convert the nitrocompounds into amines or hydroxylamines. The trimellitic acid is then recovered and dehydrated to form the highly pure trimellitic anhydride and then distilled. One of the reduction methods involves treatment with acid in the presence of iron or ferrous chloride. Hydrogen peroxide is not used.